The invention pertains to a method and a device for simultaneously measuring the mass and the height of a body, wherein the body mass is determined by measuring the deformation of at least one deformable element, wherein a scale platform is lowered, and the height measurement is realized as an absolute distance measurement to the scale platform, with the result that the distance measurement suffers from an error equal to the amount by which the scale platform has been lowered.
It is known that scales are used to determine the weight of subjects or objects. Physically, the weight of a body is taken as the force exerted on it by gravity, i.e., its weight force, which experiences gravitational acceleration. Mass could be measured directly by determining the volume and the specific density of this body. In many cases, however, a scale determines the weight of a body by determining its weight force as an elastic deformation measurement. According to this measurement method, which many scales adopt, the elastic deformation distance of a spring, resulting from the weight force acting on it, is determined versus its more-or-less constant, i.e., linear, spring characteristic. The mass is then obtained by dividing the spring constant by the deformation distance. This also means that a weight determination usually involves working with a deformation distance of a spring or of some other elastic object with a spring constant.
In modern scales, at least one load cell is often used, which is arranged under a support surface. The support surface, i.e., the scale platform, serves to hold and support the body to be measured, and during a weight measurement, it is lowered by a distance equivalent to the deformation of the load cell.
One problem with the use of devices which offer both a weighing function and a height measuring device and which are intended to determine both values simultaneously is to be found in the movement of the support surface of the scale platform from its original position to a lower one. Under the assumption that the height measurement is not being made relative to the scale platform, the lowering of the scale platform means that the height measurement suffers from an error equal to the distance by which the platform is lowered.
In principle, a quasi-systematic height measurement error caused by the lowering of the scale platform could be compensated by conducting the height measurement relative to the scale platform. This could be realized by mounting the height-measuring device on the scale platform, so that the height measuring device is lowered to the same extent as the platform. With this structural configuration, however, the at least one load cell, which is an elastic element, would necessarily have to absorb all of the forces and moments introduced into the scale platform from the upper structure, and therefore adequate stability is not always guaranteed.
An additional problem is that the conventional scale platforms are usually not large enough to support both the body to be measured and the height-measuring device. It is thus difficult to find an easy way to realize a height measurement relative to the scale platform. What must be taken into account, therefore, is the change in the distance between the scale platform and the height-measuring means, usually a headpiece, as a result of the weight force of the body acting on the platform. Measurement errors on the order of several millimeters can thus occur. Measurement errors of more than a centimeter are also possible.